Not Applicable.
The present embodiments relate to wireless communications systems and are more particularly directed to synchronizing a receiver to a transmitter in response to unevenly time-spaced synchronization signals between the transmitter and receiver.
Wireless communications have become prevalent in business, personal, and other applications, and as a result the technology for such communications continues to advance in various areas. One such advancement includes the use of spread spectrum communications, including that of code division multiple access (xe2x80x9cCDMAxe2x80x9d). In such communications, a user station (e.g., a hand held cellular phone) communicates with a base station, where typically the base station corresponds to a xe2x80x9ccell.xe2x80x9d More particularly, CDMA systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. This unique code is matched with a code of a selected user station within the cell to determine the proper recipient of a data signal.
CDMA continues to advance along with corresponding standards that have brought forth a next generation wideband CDMA (xe2x80x9cWCDMAxe2x80x9d). WCDMA includes alternative methods of data transfer, one being time division duplex (xe2x80x9cTDDxe2x80x9d) and another being frequency division duplex (xe2x80x9cFDDxe2x80x9d). The present embodiments apply by way of example to TDD and it is further introduced here. TDD data are transmitted as quadrature phase shift keyed (xe2x80x9cQPSKxe2x80x9d) symbols in data packets of a predetermined duration or time slot within a frame. By way of illustration, such a prior art frame FR is shown in FIG. 1. Frame FR is a fixed duration, such as 10 milliseconds long, and it is divided into equal duration slots. In the past it was proposed in connection with the 3G standard that the number of these equal duration slots equals 16, while more recently the standard has been modified such that each frame includes 15 equal duration slots. Each of the 15 slots has a duration of approximately 667 microseconds (i.e., 10/15 milliseconds). For the sake of reference, 15 such slots are shown in FIG. 1 as SL1 through SL15, and slots SL1 and SL8 are expanded by way of examples to illustrate additional details. Within each TDD frame FR, bi-directional communications are permitted, that is, one or more of the slots within a frame may correspond to communications from a base station to a user station while other slots in the same frame may correspond to communications from a user station to a base station.
To accomplish the communication from a user station to a base station the user station must synchronize itself to a base station. This synchronization process is sometime referred to as acquisition of the synchronization channel and is often performed in various stages. The synchronization channel, shown in expanded form as SCH in FIG. 1, includes two codes, namely, a primary synchronization code (xe2x80x9cPSCxe2x80x9d) and a secondary synchronization code (xe2x80x9cSSCxe2x80x9d), as transmitted from a base station. The PSC is presently a 256 length pseudo-noise (xe2x80x9cPNxe2x80x9d) code. As shown in frame FR of FIG. 1 and by way of example of one TDD mode, both the PSC and SSC are included and transmitted in two slots for frame FR, namely, the first slot SL1 and the eighth slot SL8. Moreover, for each slot SL1 and SL8 containing the PSC and SSC, those codes may be offset by some period of time, Toffset, within the slot. Under the present standard, Toffset is the same for both the PSC and the SSC. However, in alternative implementations, the PSC and SSC may be offset from one another, in which case it may be stated that the PSC has an offset Toffset1 from the slot boundary and the SSC has an offset Toffset2 from the slot boundary. For the sake of an example in the remainder of this document, assume that Toffset1=Toffset2. The PSC is transmitted with the same encoded information for numerous base stations while each base station group transmits a unique SSC. The actual base station is identified from the third stage of the synchronization process, which may involve correlating with the midamble (in TDD) or long code (in FDD) from the base station transmissions depending on the type of communication involved. The synchronization process typically occurs when a user station is initially turned on and also thereafter when the user station, if mobile, moves from one cell to another, where this movement and the accompanying signal transitions are referred to in the art as handoff. Synchronization is required because the user station does not previously have a set timing with respect to the base station and, thus, while slots are transmitted with respect to frame boundaries by the base station, those same slots arrive at the user station while the user station is initially uninformed of the frame boundaries among those slots. Consequently, the user station typically examines one frame-width of information (i.e., 15 slots), and from that information the user station attempts to determine the location of the actual beginning of the frame (xe2x80x9cBOFxe2x80x9d), as transmitted, where that BOF will be included somewhere within the examined frame-width of information. Further in this regard, the PSC is detected in a first acquisition stage, which thereby informs the user station of the periodic timing of the communications, and which may further assist as detailed later to identify the BOF. The SSC is detected in a later acquisition stage, which thereby informs the user station of the data location within the frame. Further, once the user station has detected a unique base station SSC, the user station also may identify the long code/midamble that is also unique to, and transmitted by, the base station, and following that determination a specific long code/midamble from that group is ascertained and which is then usable by the user station to demodulate data received in frames from the base station.
Returning now to frame FR in general and by way of particular focus to the preferred embodiments described later as well as the state of the art, note that each SCH is asymmetrically located within frame FR. More particularly, six non-synchronization slots follow the SCH in slot SL1 while seven non-synchronization slots follow the SCH in slot SL8. In other words, the location of the SCH (i.e., codes PSC and SSC) is unevenly spaced within frame FR. This asymmetry poses an issue to be addressed by the preferred embodiments, which is further appreciated by first looking to the previous 3G standard as discussed below.
Under the prior 3G standard, where recall there were 16 slots in a frame, then the SCH, as transmitted, also was located in the first and eighth slots of the frame. In order to locate these two SCH occurrences in the prior art, a user station could continuously sample 16 slots of received information and perform a PSC correlation on those samples, and by averaging those correlations to eliminate noise the synchronization channel would appear at the same slot locations within the average. For example, this technique may be implemented by applying the received information to a matched filter having the 256 length PN code of the PSC as coefficients to the filter. In this approach, the average peaks over time of those correlations correspond to the location of the synchronization channel within the collected information. However, while this approach locates the two SCH slots as corresponding to peaks within a sample of 16 slots, there is still an ambiguity whether a given peak corresponds to the originally-transmitted first or eighth slot within the frame. Thus, additional processing is required to resolve this ambiguity. Further, with the change of the 3G standard to an odd number (e.g., 15) of slots per frame, the above-described asymmetry is created. Thus, due to these factors, and also due to the lack of known timing between a transmitter and a receiver, the prior art approach does not provide a workable PSC acquisition for present applications.
In view of the above, there arises a need to provide an approach for acquisition of the PSC located asymmetrically within a wireless communication frame, as is achieved by the preferred embodiments discussed below.
In the preferred embodiment, there is a method of operating a wireless receiver. The method receives a wireless communicated signal, wherein the signal comprises asymmetrically spaced synchronization channel components. The method also defines a set of signals from the communicated signal, wherein the set spans a number of equal duration time slots and comprises at least a first synchronization channel component and a second synchronization channel component. The method also forms a first signal combination by combining a first portion of the set of signals with a second portion of the set of signals, and it forms a second signal combination by combining a third portion of the set of signals with a fourth portion of the set of signals. Finally, the method detects a location of the first synchronization channel component and a location of the second synchronization channel component in response to at least one of the first and second signal combinations. Other circuits, systems, and methods are also disclosed and claimed.